The optical waveguides or fibers used to transmit signals in optical telecommunication systems are characterized, in part, by the vector property of polarization mode dispersion (PMD). Polarization mode dispersion occurs as a result of birefringence in the fiber, which may be caused by physical asymmetry in the fiber construction itself, or by stress, strain, or other external forces imposed on the fiber. Optical fibers display an anisotropy in the refractive index, which will vary as a function of position along the fiber and as a function of time. In addition, random polarization coupling can occur, giving rise to time-varying birefringence statistics. Consequently, different polarization components of an optical signal will propagate at different velocities, resulting in a differential group delay (DGD) between the components, and causing significant broadening of the optical pulses propagating along long lengths of fiber.
PMD is widely recognized as a potential limiting factor in high-speed optical telecommunication systems. In particular, PMD is a statistical quantity that changes with time in response to environmental conditions. In order to mitigate the effects of DGD and prevent service outage due to rising DGD or a bad launch polarization state, therefore, it is important to monitor PMD itself, as well as its impact on the system. This type of a measurement implies that the output state of polarization is measured at different frequencies. The measurement is typically done by scanning a tunable laser that is coupled to a dark fiber and characterizing the polarization of the optical signal output from the fiber as a function of frequency. Dark fiber is optical fiber that is in place but not currently in use as a carrier of information in an optical telecommunication system. The conventional PMD measurements are done on dark fiber. Otherwise, a service interruption would be necessary if the tunable laser for PMD measurements were introduced into a working channel.
It is preferable to measure the PMD in an active or working channel, rather than in a dark fiber, to accurately characterize the performance of an optical telecommunication system at a given time and frequency. Methods to measure PMD in working high-speed systems have been proposed, therefore, which use the optical spectrum of the transmitter modulated signal as a probe, and apply an optical frequency filter at the output of the fiber or active channel to characterize the PMD. Conventional 10 Gb/s systems currently in use, however, are characterized by a relatively narrow modulation spectrum. Measuring the PMD with adequate optical frequency resolution using conventional means is consequently difficult, and generally limited by the resolution of available optical frequency filters.
A method and apparatus are needed, therefore, for quickly and accurately measuring the PMD in an active fiber optic channel without interruption of service.